Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Receivables, allowance for doubtful accounts	$ 8	$ 7	[1]
Aircraft fuel, spare parts and supplies, obsolescence allowance	412	354	[1]
Intangibles, accumulated amortization	$ 1,380	$ 1,313	[1]
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01	[1]
Common shares, authorized (in shares)	1,000,000,000	1,000,000,000	[1]
Common shares, outstanding (in shares)	269,914,769	286,973,195	[1]
United Airlines, Inc.
Receivables, allowance for doubtful accounts	$ 8	$ 7	[2]
Aircraft fuel, spare parts and supplies, obsolescence allowance	412	354	[2]
Intangibles, accumulated amortization	$ 1,380	$ 1,313	[2]
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01	[2]
Common shares, authorized (in shares)	1,000	1,000	[2]
Common shares, issued (in shares)	1,000	1,000	[2]
Common shares, outstanding (in shares)	1,000	1,000	[2]

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.

[2] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.